1. Field of the Invention
The present invention relates generally to semiconductor fabrication, and, more specifically, the present invention relates to the fabrication of isolation structures. In particular, the present invention relates to the fabrication of shallow trench isolation structures.
2. Description of Related Art
In the fabrication of semiconductor isolation trench structures, processing is carried out to prevent problems that become more pronounced as miniaturization progresses. One such problem occurs in the 0.13 micron geometry processing regime, and at smaller geometries. At these geometries, a high-density plasma (HDP) physical vapor deposition (PVD) shallow trench fill process may be necessary to get a trench to properly fill. Unfortunately, the requisite HDP PVD conditions result in a compressive stress within the trench that is damaging to the semiconductive channel that it borders. The stress is presumed to cause an unacceptable decrease in the maximum drive current of the device. This decrease in the maximum drive current is presumed to be caused by lattice disruptions that result from the compressive stress. Under such conditions, an isolation trench filled with this compressive material exhibits destructive or damaging compression of the transistor channel. For example, an n-metal oxide silicon (NMOS) transistor experiences degradation of the NMOS transistor performance metrics such as mobility.
One proposed solution is to form the trench fill under conditions that make the material less compressive. However, this solution may create an unacceptable trade-off that results in an unfilled trench and possibly in losing favorable wet-etch, edge leakage, and polish performance of the more compressive HDP PVD shallow trench fill recipes.
Another problem is the formation of sharp top corners at the precipice of the trench in the silicon substrate. Sharp corners of the trench may carry stronger electromagnetic fields that may cause problems when later forming active regions on either side of the trench. For example, when forming a transistor adjacent to the trench a gate insulating oxide layer is grown over the substrate and over the trench, because of the sharp corners, the gate oxide layer cannot be grown with a uniform thickness because it becomes too thin over the sharp corners. The thin gate oxide layer may break down if subjected to high electromagnetic field.
Sharp top corners also cause a problem when filling the trench. As stated above, the trench is generally filled using HDP CVD techniques that fill the trench with materials such as an oxide, polysilicon, or a combination thereof. HDP CVD processes subject the structure to plasma that also induces an electric field around the sharp corners that causes a non-uniform deposition process and that may create gaps or voids in the trench fill.
What is needed is a method of shallow trench formation that overcomes the problems of the prior art.